Development of the brain is extraordinarily complex and requires the precise coordination of both cellular and molecular processes. The establishment of dorsal-ventral and rostral-caudal axes is an essential step in early brain formation. Disturbances in this process can result in Mental retardation and brain malformations. In fact, holoprosencephaly (HPE), a severe brain anomaly, can arise from abnormalities in genes regulating dorsal-ventral patterning. Haploinsufficiency for Sonic Hedgehog (Shh), a secreted factor which induces ventral identity of the brain, causes HPE in humans. In addition, overexpression of bone morphogenetic protein 5 (BMP5), a dorsal brain inducer, also results in HPE. These data indicate that HPE arises from aberrant dorsal-ventral specification. This proposal seeks to examine the mechanisms that determine dorsal- ventral axis formation in the, vertebrate forebrain and their relationship to the pathogenesis of human brain malformations. Since loss ot expression of Shh and ectopic expression of BMP5 both result in HPE, we hypothesize that mutations affecting other genes in these pathways will also disrupt normal patterning and lead to HPE. We will test this hypothesis in two ways. First, the mechanisms of BMP5 action in the forebrain will be examined by studying the function of BMP receptors (BMPRs) in the chick embryo, an established model of brain development. Constitutively active and dominant negative BMPR constructs will be expressed in the chick brain and the resulting morphologic, cellular, and genetic consequences will be analyzed. Second, since mutations in Shh are detected in only a fraction of HPE cases, mutation screening of several genes in the Shh and BMP pathways in individuals with HPE will be conducted. This proposal describes a five-year training program in which the applicant will acquire the skills and experience required of an independent physician scientist. The primary focus throughout the grant period will be laboratory benchwork, complemented by coursework, journal clubs, and limited clinical responsibilities relevant to the project. The expertise of the mentors will provide broad and in-depth training in two areas: an animal experimental model of development, and analysis of human genes. The proposed studies will enable the candidate to transition into an independent investigator capable of integrating mutation analysis, functional developmental studies, and clinical diagnosis in future studies of the molecular basis of development and human genetic diseases.